Tailored Arrivals: Idling Down to the Final Approach

Transcontinental and oceanic pilots, who have used various techniques to conserve fuel or clip minutes off a long trip, are always disappointed to see those savings evaporate in traffic delays, as they near their destinations. "Even though we ensure we have lots of reserves," says one trans-Pacific Boeing 747 captain, "I just hate watching most of the fuel we've saved so often getting burned up while we are progressively stepped down to lower altitudes, held, vectored and finally sequenced to land during the last 30 minutes of the flight."

But at those times, not just the pilots' professional pride is hurt. For their air carrier employers, extra minutes of delay quickly convert over time into millions of dollars lost, never to be recovered. And while fuel is a major cost driver, second only to labor, a wide variety of other, lesser costs arises from the knock-on effects of delays, including incremental airframe operating costs and their time-related maintenance schedules. More recently, many airlines expect to be affected by new European Union (EU) regulations that impose drastically increased passenger compensation liability for delays. And on the social side, certain types of flight arrival procedures are noisier than others, while their associated engine emissions can have widespread environmental impacts.

Since the impacts from delays of larger, heavier aircraft are more significant than those of smaller ones, flight operations and air traffic control (ATC) specialists have focused on the former class, examining techniques to minimize these delays and their ill effects.

Idle Engine Concept

Specialists have agreed that maximum, economic traffic flow management and environmental benefits would result from procedures that allow aircraft to descend along dedicated and uninterrupted paths from their cruising altitudes, many miles distant, to a point on the final runway approach under minimum, idle engine thrust conditions. Over the past several years, much theoretical and simulation work, along with limited proof-of-concept flight tests, has gone into validating the overall proposition and, in 2004, two separate, yet quite independent sets of flight trials were undertaken in Australia and the United States.

The Australian trials--known as tailored arrivals (TAs)--were undertaken by the team of Airservices Australia, Qantas, Boeing and the European Air Traffic Alliance, and are an adaptation of an idle thrust descent concept originally proposed by Boeing when it introduced its Air Traffic Management (ATM) Division in 2001. (That division subsequently became part of Boeing's Phantom Works activity.)

Bob Peake, head of ATM at Airservices, summarizes the project as "a way to see what could be usefully achieved, utilizing the current capabilities of existing aircraft flight management computers (FMCs) to provide an enhanced and more predictable ATM arrivals environment." The concept, he says, is that, to meet a computed time at an entry fix, the arrival clearance is "tailored" to the specific aircraft type's optimal descent trajectory at the current gross weight, and takes into account the current winds and temperatures at various levels, the current traffic disposition, and all necessary speed, altitude and routing constraints. The clearance is then data-linked to the aircraft before top of descent (TOD).

The trials are planned over three phases, the first of which was flown between April and September of 2004 by two Qantas B747s and two Qantas Airbus A330s on scheduled operations. Phase 1 essentially comprised a series of interface shakedown, operator familiarity and proof-of-concept tests, followed by analysis. Approximately 80 flights were conducted--the 747s flying between Singapore and Sydney, and the A330s between Perth and Melbourne. For the purpose of the trial, all ground communications originated exclusively from The Australian Advanced Air Traffic System (TAAATS). In operation the ATC traffic comprised only data communications between the ATC center and each individual aircraft's FMC: no voice transmissions were involved.

Australian Trials

For this phase the project used the already existing and readily available future air navigation system (FANS) 1/A data link network, the two-way communications medium between aircraft and air traffic control centers that is widely implemented by air carriers across the Pacific and elsewhere. Using FANS meant no new equipment was required aboard the trials aircraft, crews already were trained in its use, and potentially, a very large number of already equipped aircraft could eventually use the system with no additional costs.

The uplinked tailored descent paths, whose top of descent point and actual route varied with aircraft and weather conditions, were then connected within the FMC to establish standard arrival procedures for the appropriate runway at Sydney or Melbourne, thereby providing a seamless transition into the terminal area and thence on to the final approach.

Phase 1 results were impressive. Prior to TOD, when the aircraft was around 40 minutes from landing, the FMC used the uplinked clearance to predict the arrival time at the terminal area entry fix. The predictions were accurate to within as little as two seconds, and never more than 30 seconds of the actual arrival times. A limited number of tests to assess the ability to gain or lose time also were performed. When instructed to change ATC's previously requested required time of arrival (RTA), the FMC was required, from distances out between 175 and 245 nm, to gain or lose time at the terminal area entry fix. Preliminary results were encouraging, but more work will be carried out in this area in phases 2 and 3. Each uplinked procedure to the FMC also contained alternate arrival routes that could be used to guide the aircraft out of the procedure at predefined descent points, to allow time to be gained or lost, according to subsequent ATC needs.

Phase 2 will examine more complex clearances, assess compatibility with other, non-FANS data links, and study the need for improvements in the controller/system interface. A key element in phase 2 will be development of a tailored arrival prototype clearance generator to produce continuous, dynamic clearances, rather than the single, precreated clearances used in phase 1. Phase 3 will expand the system's multiple-aircraft arrival capabilities, including further work on integrating non-FANS aircraft. It also will study integrating the tailored arrivals with tactical flow and metering systems, along with any necessary enhancements to controller tools.

But the trials have shown the tailored arrival concept's promise: the phase 1 flight and simulation data is expected to show savings of between 400 and 800 pounds (181 to 363 kg) of fuel per flight, potentially adding up to more than $100,000 per year per aircraft.

In the States

In the United States, a noise abatement procedures working group, headed by the Massachusetts Institute of Technology (MIT) and including Boeing, FAA, NASA and the Louisville, Ky, Regional Airport Authority, established a trials program with freight carrier United Parcel Service (UPS), whose home base and main hub are in Louisville.

As the group's name suggests, the primary aim of these trials was noise abatement and lessened environmental impact since, unlike the approaches to Sydney and Melbourne, the approaches to Louisville from any direction are over relatively heavily populated areas. Like most air freight operations, moreover, virtually all of the carrier's operations are conducted late at night or in the early morning.

Test Results

In 2002 UPS initiated its own evaluation of idle thrust noise abatement procedures for freighters arriving in Louisville from the West Coast at night. In that trial two B767s flew the same lateral descent path consecutively into Louisville each night over a two-week period. One flew a standard descent with typical intermediate leveling-off steps, and the other flew an idle-power, continuous descent approach (CDA) procedure. Substantial (up to 6-dB) noise reductions were achieved from the CDA, along with a 30 percent reduction in nitrous oxide (NOx) emissions below 3,000 feet plus a 500-pound (227-kg) reduction in fuel burn and a 100-second saving in flight time.

These tests were significantly expanded in September 2004, in a two-week trial organized by the MIT-led group. The number of aircraft was increased to 12 to 14 each night, including a mix of B757s and 767s, and a total of 125 CDAs were flown. While noise, emissions, fuel burn and time savings were measured and are being analyzed, initial results indicate significant reductions in noise--and therefore in the overall ground area affected by noise--and in engine emissions, fuel burn and descent procedure time.

Fuel reduction through the descent fell by 364 pounds (165 kg) for the B767 and 118 pounds (54 kg) for the B757, while the descent procedure time decreased by 147 seconds for the B767 and 118 seconds for the B757.

Two Philosophies

The overall outcome of both the Australian and American trials therefore can be regarded as fully endorsing the idle thrust descent concept. However, each trial used quite different technical philosophies towards defining and controlling the actual descent path. As mentioned, the Australian data link-based tailored arrival process takes aircraft and wind data downloaded from the FMC prior to the estimated top of descent. It then data-links back up an assigned TOD point, along with a descent path linked with a standard terminal arrival procedure already stored in the FMC. There are no crew/ATC voice exchanges.

In the American CDA process the descent path is laterally fixed--and permanently stored in the FMC--right through to final approach and touchdown. The TOD is the key variable. This is verbally assigned to the crew from either the air route traffic control center (ARTCC) or the terminal radar approach control (TRACON), which is then responsible for separations, usually 15 minutes in trail. But crew/ATC voice exchanges typically are reduced by more than 60 percent.

Although the Australian and U.S. processes show tremendous potential for the future, they are still quite unique--that is, technically incompatible--although, as noted, Airservices Australia expects to address this in later phases. Nevertheless, Australia appears firmly committed to a FANS-compatible environment, while the United States is more likely to support an FMC/voice/ARTCC approach similar to what was done in the Louisville tests.

But here FAA faces a "Hobson's choice," i.e., no choice at all. Having postponed its own controller pilot data link communications (CPDLC) project until after the implementation of the en route automation modernization (ERAM) program, the United States could conceivably not have an operational nationwide data link service until 2015. NASA, however, continues to explore a FANS-compatible U.S. implementation. NASA Ames is understood to be considering a FANS-based tailored arrival-like evaluation later this year or in 2006.

Europe has yet to announce its philosophy regarding controlled, idle-thrust descent procedures, although a number of limited test activities have taken place in the UK. A larger continental initiative is expected to commence in 2006. The European position will be an interesting one since, under Eurocontrol's Link 2000+ program, more than 100 aircraft are equipping with FMCs built to aeronautical telecommunications network (ATN) data link protocols, which are at present technically incompatible with each of the other two concepts.

The Future

Eventually, of course, international standardization will require technology compatibility, both on the ground and in the air, to achieve seamless worldwide controlled descent operations. Today's regional developments underline the potential of the concept, but different regional solutions will be unacceptable in the future.

Yet technology development is far from being the only factor. While the descent technique shows savings in operating costs, it is not yet clear whether the investment in fleetwide purchase, installation and certification costs of new equipment--even if it were available today--would provide hard-pressed air carriers with the payback level they must now demand. Conversely, air traffic service providers, faced with ever increasing airspace demands, may find it necessary in the future to consider equipment mandates, or at least priority aircraft handling, to ensure maximum traffic flows in high-density regions. And, slowly approaching, are the proposed aviation emission and noise constraints associated with the Kyoto Accord. While one can therefore state with confidence that controlled descent procedures are inevitable, putting a date on that inevitability may be the most difficult task of all.

At A Glance

Idle-thrust arrivals employing dedicated and uninterrupted descent paths from cruise altitudes promise to reduce air traffic control delays, saving carriers time and money. Trials in Australia and the United States, using different technologies, have demonstrated feasibility, but compatibility issues remain.